We have studied the pressure-induced quantum phase transition in the singlet-ground-state antiferromagnet ${\mathrm{CsFeCl}}_3$. Neutron diffraction experiments under pressure evidence the magnetic long-range order at low temperatures. Magnetic structure analysis reveals a ${120}^{\circ }$ structure with a propagation vector of ${\mathit{k}}_{\mathrm{mag}}=(1/3,1/3,0)$. The estimated critical exponent of the order parameter suggests that ${\mathrm{CsFeCl}}_3$ belongs to the universality class of $U\left(1\right)\times Z_2$ symmetry which is expected to realize the chiral liquid state.

In this Rapid Communication, we propose a mechanism for electric dipole active spin resonance caused by spin-dependent electric polarization in a quantum spin gapped system. This proposal was successfully confirmed by high-frequency electron spin resonance (ESR) measurements of the quantum spin dimer system ${\mathrm{KCuCl}}_3$. ESR measurements by an illuminating linearly polarized electromagnetic wave reveal that the optical transition between the singlet and triplet states in ${\mathrm{KCuCl}}_3$ is driven by an ac electric field. The selection rule of the observed transition agrees with the calculation by taking into account spin-dependent electric polarization. We suggest that spin-dependent electric polarization is effective in achieving fast control of quantum spins by an ac electric field.

Materials exhibiting controllable magnetic phase transitions are currently in demand for many spintronics applications. Here, we investigate from first principles the electronic structure and intrinsic anomalous Hall, spin Hall, and anomalous Nernst response properties of the FeRh metallic alloy which undergoes a thermally driven antiferromagnetic-to-ferromagnetic phase transition. We show that the energy band structures and underlying Berry curvatures have important signatures in the various Hall effects. Specifically, the suppression of the anomalous Hall and Nernst effects in the antiferromagnetic state and a sign change in the spin Hall conductivity across the transition are found. It is suggested that the FeRh can be used as a spin current detector capable of differentiating the spin Hall effect from other anomalous transverse effects. The implications of this material and its thermally driven phases as a spin current detection scheme are also discussed.

A general procedure is developed to investigate the elastic response and calculate the elastic constants of stressed and unstressed materials through continuum field modeling, particularly the phase-field crystal (PFC) models. It is found that for a complete description of system response to elastic deformation, the variations of all the quantities of lattice wave vectors, their density amplitudes (including the corresponding anisotropic variation and degeneracy breaking), the average atomic density, and system volume should be incorporated. The quantitative and qualitative results of elastic constant calculations highly depend on the physical interpretation of the density field used in the model, and also importantly, on the intrinsic pressure that usually pre-exists in the model system. A formulation based on thermodynamics is constructed to account for the effects caused by constant pre-existing stress during the homogeneous elastic deformation, through the introducing of a generalized Gibbs free energy and an effective finite strain tensor used for determining the elastic constants. The elastic properties of both solid and liquid states can be well produced by this unified approach, as demonstrated by an analysis for the liquid state and numerical evaluations for the bcc solid phase. The numerical calculations of bcc elastic constants and Poisson's ratio through this method generate results that are consistent with experimental conditions, and better match the data of bcc Fe given by molecular dynamics simulations as compared to previous work. The general theory developed here is applicable to the study of different types of stressed or unstressed material systems under elastic deformation.

Enhancing the optical response of graphene is a topic of interest with applications in optoelectronics. Subject to light irradiation, graphene can exhibit nontrivial topologically insulating states, effectively turning itself into a Floquet topological insulator due to the time periodicity of the external driving. We find that, when random disorder is present, its interplay with the topologically insulating states can have a dramatic effect on electronic transport through graphene. In particular, we consider the prototypical setting where a graphene nanoribbon is irradiated by circularly polarized light, where the length of the nanoribbon is sufficiently long so that evanescent states have little effect on transport. We uncover a resonance phenomenon in which the conductance is enhanced as the disorder strength is increased from zero, reaches a maximum value for an optimal level of disorder, and decreases as the disorder is strengthened further. With respect to its value at the zero-disorder strength, the maximum conductance value can be as much as $50\%$ higher. Qualitatively, this can be understood as a result of the dynamical interplay between disorder and Floquet states (channels) generated by light irradiation. Quantitatively, the resonance phenomenon can be explained in the framework of Born theory, where the disorder reorganizes the Floquet Hamiltonian and enhances the effective coupling between the adjacent Floquet conducting channels. That is, disorder is capable of promoting both photon absorption and emission, leading to significant enhancement of nonequilibrium electronic transport. We demonstrate the robustness of the resonance phenomenon by investigating the effects of spatial symmetry breaking on transport and provide an understanding based on analyzing the behavior of the density of states of the Floquet channels.

Magnetization and specific-heat measurements on a $\mathrm{UIrS}{\mathrm{i}}_3$ single crystal reveal Ising-like antiferromagnetism below $T_{\mathrm{N}}=41.7\mathrm{K}$ with the easy magnetization direction along the $c$ axis of the tetragonal structure. The antiferromagnetic ordering is suppressed by magnetic fields $>H_{\mathrm{c}}$ (${\mu }_0{H}_{\mathrm{c}}=7.3\mathrm{T}$ at 2 K) applied along the $c$ axis. The first-order metamagnetic transition at $H_{\mathrm{c}}$ exhibits asymmetric hysteresis reflecting a slow reentry of the complex ground-state antiferromagnetic structure with decreasing field. The hysteresis narrows with increasing temperature and vanishes at 28 K. A second-order metamagnetic transition is observed at higher temperatures. The point of change of the order of transition in the established H-T magnetic phase diagram is considered as the tricritical point (at $T_{\mathrm{tc}}=28\mathrm{K}$ and ${\mu }_0{H}_{\mathrm{tc}}=5.8\mathrm{T}$). The modified-Curie-Weiss law fits of temperature dependence of the $a$- and $c$-axis susceptibility provide opposite signs of Weiss temperatures, ${\mathrm{\Theta }}_{\mathrm{p}}^a\sim -51\mathrm{K}$ and ${\mathrm{\Theta }}_{\mathrm{p}}^c\sim +38\mathrm{K}$, respectively. This result and the small value of ${\mu }_0{H}_{\mathrm{c}}$ contrasting to the high $T_{\mathrm{N}}$ indicate competing ferromagnetic and antiferromagnetic interactions responsible for the complex antiferromagnetic ground state. The simultaneous electronic-structure calculations focused on the total energy of ferromagnetic and various antiferromagnetic states, the U magnetic moment, and magnetocrystalline anisotropy provide results consistent with experimental findings and the suggested physical picture of the system.

Frustrated spin ladders show magnetization plateaux depending on the rung-exchange interaction and frustration defined by the ratio of first and second neighbor exchange interactions in each chain. This paper reports on its magnetic phase diagram. Using the variational matrix-product state method, we accurately determine phase boundaries. Several kinds of magnetization plateaux are induced by the frustration and the strong correlation among quasiparticles on a lattice. The appropriate description of quasiparticles and their relevant interactions are changed by a magnetic field. We find that the frustration differentiates the triplet quasiparticle from the singlet one in kinetic energy.

In this work, we report unusual observations of Kondo effect and coexistence of Kondo effect and ferromagnetism in indium tin oxide (ITO) nanowires that were synthesized without incorporating any magnetic impurities. The temperature-dependent resistivity ($\rho -T$) data exhibited an upturn below 80 K and then tended to saturate below 10 K. The $\rho -T$ and magnetoresistance data were analyzed using the $n$-channel Kondo model, and from the obtained values of $\mathit{S}=1$ and $n\sim 1$, the nanowires were expected to be an underscreened Kondo system. A model was also proposed to explain the formation of localized $\mathit{S}=1$ spin centers in the ITO nanowires. This work could provide insights into the understanding of spin-related novel phenomena in metal oxide nanostructures.

The results of x-ray diffraction and ${}^{57}\mathrm{Fe}$ and ${}^{151}\mathrm{Eu}$ Mössbauer spectroscopy measurements, supplemented with ab initio hyperfine-interaction parameter calculations, on the new 35.5 K superconductor ${\mathrm{Rb}}_{1-\delta }{\mathrm{EuFe}}_4{\mathrm{As}}_4$ are presented. The superconductor crystallizes in the tetragonal space group $P4/mmm$ with the lattice parameters $a=3.8849\left(1\right)$ Å and $c=13.3370\left(3\right)$ Å. It is shown that there is no magnetic order of the Fe magnetic moments down to 2.1 K and that the ferromagnetic order is associated solely with the Eu magnetic moments. The Curie temperature $T_{\mathrm{C}}=16.54\left(8\right)$ K is determined from the temperature dependence of both the hyperfine magnetic field at ${}^{151}\mathrm{Eu}$ nuclei and the transferred hyperfine magnetic field at ${}^{57}\mathrm{Fe}$ nuclei that is induced by the ferromagnetically ordered Eu sublattice. The Eu magnetic moments are demonstrated to be perpendicular to the crystallographic $c$ axis. The temperature dependence of the principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described by a $T^{3/2}$ power-law relation. Good agreement between the calculated and measured hyperfine-interaction parameters is observed. The Debye temperature of ${\mathrm{Rb}}_{1-\delta }{\mathrm{EuFe}}_4{\mathrm{As}}_4$ is found to be 391(8) K.

We report on the superconducting properties of ${\mathrm{Nb}}_{1-x}{\mathrm{Ti}}_x\mathrm{N}$ thin films of thickness $\sim 10$ nm, with different Ti fraction $x$ in the range $0\le x\le 0.5$, deposited by high-temperature chemical vapor deposition. In this parameter range, we observe that the superconducting critical temperature ($T_c$) increases with $x$. We analyze the possible role played by various parameters—Debye temperature, electron-phonon coupling constant, and Coulomb pseudopotential—in determining the observed variation of $T_c$ with $x$.

We analyze quantum fluctuation effects at the onset of charge or spin density wave order in two-dimensional metals with an incommensurate $2k_F$ wave vector connecting a single pair of hot spots on the Fermi surface. We compute the momentum and frequency dependence of the fermion self-energy near the hot spots to leading order in a fluctuation expansion (one loop). Non-Fermi liquid behavior with anomalous frequency scaling and a vanishing quasiparticle weight is obtained. The momentum dependence yields a divergent renormalization of the Fermi velocity and a flattening of the Fermi surface near the hot spots. Going beyond the leading-order calculation, we find that the one-loop result is not self-consistent. We show that any momentum-independent self-energy with a non-Fermi liquid frequency exponent wipes out the peak of the polarization function at the $2k_F$ wave vector, and thus destroys the mechanism favoring $2k_F$ density waves over those with generic wave vectors. However, a $2k_F$ density wave quantum critical point might survive in presence of a sufficiently flat renormalized Fermi surface.

$\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ belongs to the family of layered strontium ruthenates and exhibits a range of unusual emergent properties, such as electron nematic behavior and metamagnetism. Here, we show that epitaxial film strain significantly modifies these phenomena. In particular, we observe enhanced magnetic interactions and an electron nematic phase that extends to much higher temperatures and over a larger magnetic-field range than in bulk single crystals. Furthermore, the films show an unusual anisotropic non-Fermi-liquid behavior that is controlled by the direction of the applied magnetic field. At high magnetic fields, the metamagnetic transition to a ferromagnetic phase recovers isotropic Fermi-liquid behavior. The results support the interpretation that these phenomena are linked to the special features of the Fermi surface, which can be tuned by both film strain and an applied magnetic field.

Increasing the quantum efficiency (QE) of metal photocathodes is in the design and development of photocathodes for free-electron laser applications. The growth of metal oxide thin films on certain metal surfaces has previously been shown to reduce the work function (WF). Using a photoemission model B. Camino et al. [Comput. Mater. Sci. 122, 331 (2016)] based on the three-step model combined with density functional theory calculations we predict that the growth of a finite number of MgO(100) or BaO(100) layers on the Ag(100) surface increases significantly the QE compared with the clean Ag(100) surface for a photon energy of 4.7 eV. Different mechanisms for affecting the QE are identified for the different metal oxide thin films. The addition of MgO(100) increases the QE due to the reduction of the WF and the direct excitation of electrons from the Ag surface to the MgO conduction band. For BaO(100) thin films, an additional mechanism is in operation as the oxide film also photoemits at this energy. We also note that a significant increase in the QE for photons with an energy of a few eV above the WF is achieved due to an increase in the inelastic mean-free path of the electrons.

We follow the diffusion of CO molecules on Cu(111) by time-lapsed low-temperature scanning tunneling microscopy. The diffusivity of individual CO molecules oscillates with the distance to its nearest neighbor due to the long-range interaction mediated by the surface state electrons. The markedly different wavelengths of the oscillation at a coverage of 0.6% ML as compared to the one at 6% ML coverage correspond to two different wavelengths of the surface state electrons, consistent with a shift of the surface state by 340 meV. This surprisingly large shift as compared to results of averaging methods suggests a local modification of the surface state properties.

Spin interactions between a two-dimensional electron system at the InAs surface and transition metal ions, ${\mathrm{Fe}}^{3+}$, ${\mathrm{Co}}^{2+}$, and ${\mathrm{Ni}}^{2+}$, deposited on the InAs surface, are probed by antilocalization measurements. The spin-dependent quantum interference phenomena underlying the quantum transport phenomenon of antilocalization render the technique sensitive to the spin states of the transition metal ions on the surface. The experiments yield data on the magnitude and temperature dependence of the electrons' inelastic scattering rates, spin-orbit scattering rates, and magnetic spin-flip rates as influenced by ${\mathrm{Fe}}^{3+}$, ${\mathrm{Co}}^{2+}$, and ${\mathrm{Ni}}^{2+}$. A high magnetic spin-flip rate is shown to mask the effects of spin-orbit interaction, while the spin-flip rate is shown to scale with the effective magnetic moment of the surface species. The spin-flip rates and their dependence on temperature yield information about the spin states of the transition metal ions at the surface, and in the case of ${\mathrm{Co}}^{2+}$ suggest either a spin transition or formation of a spin-glass system.

We report a systematic study of Sm valence in the prototypical intermediate valence compound ${\mathrm{SmB}}_6$. Sm mean valence $v_{\mathrm{Sm}}$ was measured by x-ray absorption spectroscopy as a function of pressure $\left(1<P<13\mathrm{GPa}\right)$ and temperature $\left(3<T<300\mathrm{K}\right)$. Pressure-induced magnetic order was detected above $P_c=10\mathrm{GPa}$ by resistivity measurements. A shift toward the localized $4f$ state with increasing $P$ and/or $T$ is evident from an increase in $v_{\mathrm{Sm}}$. However $v_{\mathrm{Sm}}$ at $P_c$ is anomalously far below 3, which differs from the general case of nonmagnetic-magnetic transition in Yb and Ce compounds. From the $T$ dependence of $v_{\mathrm{Sm}}(P,T)$, we found that $v_{\mathrm{Sm}}(P,T)$ consists of two different characteristic components: One is associated with low-energy electronic correlations involving Kondo-like behavior, and the other is associated with high-energy valence fluctuations.

In this work we demonstrate that linearized gravity exhibits gapless topological order with an extensive ground state degeneracy. This phenomenon is closely related both to the topological order of the pyrochlore $U\left(1\right)$ spin liquid and to recent work by Hawking and co-workers, who used the soft-photon and graviton theorems to demonstrate that the vacuum in linearized gravity is not unique. We first consider lattice models whose low-energy behavior is described by electromagnetism and linearized gravity, and then argue that the topological nature of these models carries over into the continuum. We demonstrate that these models can have many ground states without making assumptions about the topology of spacetime or about the high-energy nature of the theory, and show that the infinite family of symmetries described by Hawking and co-workers is simply the different topological sectors. We argue that in this context black holes appear as topological defects in the infrared theory, and that this suggests a potential approach to understanding both the firewall paradox and information encoding in gravitational theories. Finally, we use insights from the soft-boson theorems to make connections between deconfined gauge theories with continuous gauge groups and gapless topological order.

The past few years have witnessed increased attention to the quest for Majorana-like excitations in the condensed matter community. As a promising candidate in this race, the one-dimensional chiral Majorana edge mode (CMEM) in topological insulator-superconductor heterostructures has gathered renewed interests after an experimental breakthrough [Q. L. He et al., Science 357, 294 (2017)]. In this work, we study computationally the quantum transport of topological insulator-superconductor hybrid devices subject to time-periodic modulation. We report half-integer quantized conductance plateaus at $\frac{1}{2}\frac{e^2}{h}$ and $\frac{3}{2}\frac{e^2}{h}$ upon applying the so-called sum rule in the theory of quantum transport in Floquet topological matter. In particular, in a photoinduced topological superconductor sandwiched between two Floquet Chern insulators, it is found that for each Floquet sideband, the CMEM admits equal probability for normal transmission and local Andreev reflection over a wide range of parameter regimes, yielding half-integer quantized plateaus that resist static and time-periodic disorder. While it is well-established that periodic driving fields can simultaneously create and manipulate multiple pairs of Majorana bound states, their detection scheme remains elusive, in part due to their being neutral excitations. Therefore the $\frac{3}{2}\frac{e^2}{h}$ plateau indicates the possibility to verify the generation of multiple pairs of photoinduced CMEMs via transport measurements. The robust and half-quantized conductance plateaus due to CMEMs are both fascinating and subtle because they only emerge after a summation over contributions from all Floquet sidebands. Our work may add insights into the transport properties of Floquet topological systems and stimulate further studies on the optical control of topological superconductivity.

We study the filling-driven Mott transition involving the metallic and paramagnetic insulating phases in $\mathrm{SU}\left(N\right)$ Fermi-Hubbard models, using the dynamical mean-field theory and the numerical renormalization group as its impurity solver. The compressibility shows a striking temperature dependence: near the critical end-point temperature, it is strongly enhanced in the metallic phase close to the insulating phase. We demonstrate that this compressibility enhancement is associated with the thermal suppression of the quasiparticle peak in the local spectral functions. We also explain that the asymmetric shape of the quasiparticle peak originates from the asymmetry in the dynamics of the generalized doublons and holons.

It is experimentally well established that the Cr(001) surface exhibits a sharp resonance around the Fermi level. However, there is no consensus about its physical origin. It is proposed to be either due to a single particle $d_{z^2}$ surface state renormalized by electron-phonon coupling or the orbital Kondo effect involving the degenerate $d_{xz}/$ $d_{yz}$ states. In this paper we examine the electron-phonon coupling of the Cr(001) surface by means of ab-initio calculations in the form of density functional perturbation theory. More precisely, the electron-phonon mass-enhancement factor of the surface layer is investigated for the 3d states. For the majority and minority spin $d_{z^2}$ surface states we find values of 0.19 and 0.16. We show that these calculated electron-phonon mass-enhancement factors are not in agreement with the experimental data even if we use realistic values for the temperature range and surface Debye frequency for the fit of the experimental data. More precisely, then experimentally an electron-phonon mass-enhancement factor of $0.70\pm 0.10$ is obtained, which is not in agreement with our calculated values of 0.19 and 0.16. Our findings suggest that the experimentally observed resonance at the Cr(001) surface is not due to electron-phonon effects but due to electron-electron correlation effects.